1. Field of the Invention
This invention relates to a polymeric composite material produced by dispersing metal sulfide semiconductor microparticles having optical characteristics such as a nonlinear optical effect, photoelectric converting characteristics, and the like. This invention also relates to a process for manufacturing the polymeric composite material.
2. Description of the Related Art
With advanced information processing, nonlinear optical materials having a large nonlinear optical effect are under investigation to produce an optical logic device, optical switch, and the like, which constitute fundamental techniques in optical computers.
Compounds conventionally known as the nonlinear optical materials include inorganic ferroelectric substances such as LiNbO.sub.3, BaTiO.sub.3, KH.sub.2 PO.sub.4, and the like; quantum well structure semiconductors using GaAs or the like; organic monocrystals such as 4'-nitrobenzylidene-3-acetamino-4-methoxyaniline (MNBA), 2-methyl-4-nitroaniline (MNA), and the like; conjugate organic high polymers such as polydiacetylene, polyarylenevinylene, and the like; and semiconductor microparticle dispersion glass produced by dispersing CdS, CdSSe, and the like in glass. Especially, the semiconductor microparticle dispersion glass has been vigorously studied as promising nonlinear optical materials possessing both a high nonlinear optical susceptibility and high speed response since Jain and Lind discovered in the United States in 1983 that the so-called color glass filter prepared by dispersing semiconductor microparticles in glass exhibited a high tertiary nonlinear optical effect (for example, J. Opt. Soc. Am., 73, 647 (1983)).
Also, photoelectric converting devices such as the solar battery, photocatalyst, and the like have been studied with a view to utilizing clean energy. As typical materials for the photoelectric converting devices, compound semiconductors such as crystal silicon, noncrystal silicon, InGaP, CdS, and the like are known.
These materials are used either singly or after being made into a polymeric composite material by dispersing each of these materials in an inorganic high polymer, e.g. silica glass or an organic high polymer.
The following processes are proposed for manufacturing the polymeric composite material comprising metal sulfide microparticles dispersed in an inorganic high polymer:
The first example is described in J. Opt. Soc. Am., Vol. 73, 647 (1983). A mixture of glass as the dispersion medium or powder as the starting material thereof and CdS.sub.x Se.sub.1-x are melted to form a glass molten liquid. This glass molten liquid is rapidly cooled to around room temperature to obtain a solid solution. The solid solution is then reheated to an appropriate temperature for a prescribed period of time to precipitate semiconductor microparticles (the so called "molten-rapid cooling process").
This molten-rapid cooling process, however, has the problem that the semiconductor raw materials are decomposed and evaporated since it is necessary to heat the semiconductor raw materials at temperatures above 1,000.degree. C., so the appropriate sort of semiconductor and the amount of semiconductor to be added are limited.
The second process consists of the above molten-rapid cooling process or a process called the "sol-gel process" for preparing a medium by hydrolyzing silicon alkoxide or the like. Starting materials such as a metallic salt and the like are mixed to produce a polymeric composite material in which a metal salt is dispersed. The polymeric composite material is then processed either by hydrogen sulfide gas (see Abstracts of Lecture of Annual Meeting, Japan Ceramic Association, page 336, (1989); Journal of Non-Crystalline Solids, Vol. 126, 87 (1990); Japanese Patent Application Laid-Open (JP-A) Nos. 3-199137, 3-295826, and 4-274223) or by a solution containing a sulfur ion (see Japanese Patent Application Laid-Open (JP-A) Nos. 3-199137 and 4-270131).
In these processes, however, it is difficult to convert the metal salt within the polymeric composite material to metal sulfide homogeneously when the polymeric composite material including the dispersed metal salt is thick. Also, the particle diameter of the metal sulfide existing inside the polymeric composite material is different from that existing on the surface and hence the particle diameter of the metal sulfide cannot be controlled easily. Further, because hydrogen sulfide gas used in the process is highly toxic, it is operationally dangerous and hence a special safety device is required. Also, tetraalkoxysilane used in the conventional sol-gel process tends to generate cracks when drying the gel. When forming a thin film of tetraalkoxysilane on a substrate to make a device, only insufficient film thickness can be obtained. Conventionally, it is necessary to adopt a complicated process in which steps of forming a thin film with a thickness of about of 0.1 .mu.m or less on a substrate and of baking the substrate at temperatures above several hundred degrees Celsius must be repeated to prepare sufficient film thickness for the device.
A third process is the so-called dry process. To exemplify the dry process, a known process for manufacturing semiconductor microparticle dispersion glass using glass or elemental semiconductor polycrystal of SiO.sub.2, CdS, or the like or a mixture of these as targets by spattering is cited (see, for example, J. Appl. Phys., 63(3), 957 (1988); Japanese Patent Application Laid-Open (JP-A) Nos. 4-2632, 4-113334, and 4-345136). Other than the above, a process for forming a film using the CVD process is also known (see Japanese Patent Application Laid-Open (JP-A) Nos. 1-319985 and 4-345139). These processes enable semiconductors to be added in an amount larger than that possible in the molten-rapid cooling process.
In these dry processes, the apparatus is expensive and the film forming rate is low. Hence, there is the problem that a thick film cannot be formed easily, although the dry process can be utilized for forming a thin film. Also, the applicability of the device produced by the dry process is limited since the configuration of the device is limited to a thin film.
On the other hand, as the processes for preparing the polymeric composite materials comprising metal sulfide microparticles dispersed in an organic high polymer, the following processes are known. Similarly to the polymeric composite material comprising metal sulfide microparticles dispersed in an inorganic high polymer, a mixture in which a metal salt or the like is dispersed in an organic high polymer is prepared and then the mixture is processed by hydrogen sulfide gas (see Japanese Patent Application Laid-Open (JP-A) Nos. 4-229807 and 4-238304). This process is, however, carried out also using hydrogen sulfide gas, exhibiting the same problem as above.
In addition, there is still another known process including a solution reaction, in which chalcogenide semiconductor raw materials and a stabilizing agent (organic high polymer) for chalcogenide semiconductor microparticles resulted from these raw materials are allowed to coexist and the raw materials are allowed to react with a chalcogenizing agent, followed by distilling the solvents used (see Japanese Patent Application Laid-Open (JP-A) No. 5-24826). Further, a process is known in which a hydroxide of Cd is sedimented in high polymer gel by an opposed diffusion process and then an aqueous Na.sub.2 S solution is added to act on the sedimented hydroxide of Cd to convert it into CdS as described in High Polymer Theses, Vol. 47, 935 (1990). These processes, however, have the problems that a plurality of steps is required, the operations are complicated, usable high polymers are limited, and it is not easy to form a device having an optional shape. Also, there is the problem that the reaction residue of the chalcogenizing agent tends to remain as an impurity.